Ferroelectric liquid crystal displays offer great advantages in terms of quick response, the time needed for a change in orientation of the ferroelectric liquid crystal array being much shorter than the time needed for the change in a typical nematic liquid crystal. One great advantage of this quick response is that sequential coloring of pixels is possible at a refresh rate that facilitates color fusion, that is, the appearance to the human eye of the pixel as a single color rather than as rapidly sequentially changing colors. Sequential color also enables a higher resolution for a particular pixel size, as a single pixel can display all colors, rather than requiring three pixels, red, green and blue, to display a full color spectrum.
One phenomenon of liquid crystal displays such as those of the ferroelectric type is termed “image sticking,” also known as “optical hysteresis” or “ghost images,” referring to a residual image that is displayed on the screen persisting long after the driving voltages are removed from the ferroelectric liquid crystal (FLC) pixels. It is believed that ions present in the liquid crystal assembly can contribute to the image sticking problem. Charge densities are especially high in FLC devices because of the spontaneous polarization and the resulting internal electric fields. In sequential color ferroelectric liquid crystal on silicon (FLCOS) devices, a DC offset in the drive algorithm causes severe sticking, but operation in a DC balanced mode can reduce image sticking. DC balancing refers to the process wherein a voltage of inverse polarity is applied to a liquid crystal pixel immediately following application of a display voltage to assist in neutralizing residual electrical charges responsible for image sticking. However, this mode of operation requires that the LEDs supplying the light that is modulated by the FLCs be turned off during the balance phase when the inverse polarity voltage is applied, thereby reducing the light output of the device. See, for example U.S. Pat. No. 6,075,577.
Images are produced on an FLC display by applying a suitable pattern of voltages to the display's pixels and viewing the resultant pattern of FLC optical states using crossed polarizers. In standard video systems the displayed image changes at a rate of 60 frames per second. Under certain conditions an image can become “stuck” for a time; meaning that when subsequent images are displayed, the stuck image is superimposed on those later images.
Multiple mechanisms can contribute to image sticking: charge accumulation at FLC-alignment layer interfaces, changes in director orientation at the alignment layer, changes of pretilt, and perhaps changes in director gliding behavior. The mechanism addressed here is the accumulation of electrical charge at the surfaces of the FLC layer in response to applied voltages. Typically, as judged by the appearance of image sticking, the time constant for growth and decay of accumulated surface charge ranges from minutes to hours. To combat this problem FLC displays generally show each image and its complement in sequence (i.e. dark pixels made bright and vice versa). This ensures that the average voltage experienced by each pixel is zero, thus no charge should accumulate (assuming that the charge accumulation time is long compared to the frame period). The disadvantage of this scheme, referred to here as dc-compensation (see Clark, N. A., C. Crandall, M. A. Handschy, M. R. Meadows, R. M. Malzbender, C. Park, and J. Z. Xue, FLC microdisplays. Ferroelectrics, 2000, 246, p. 97-110), is that illumination must be turned off during display of the image complement so that it is not seen by viewers; the resulting 50% duty cycle reduces the effective display brightness by half. Duty cycle is defined as the proportion of time that the liquid crystal is driven so as to display the image. This is the maximum time that it is desirable to illuminate a display. For various reasons, it may not be desirable to illuminate the display for the entire period that the liquid crystal is driven so as to display the image.